In a conventional vehicle, the brake is no longer necessarily operated by means of a pneumatic control pressure; it can also be operated by an electrically transmitted signal whereby the brake pedal emits an electric signal when it is actuated by the driver. This signal is electronically processed and is used to adjust the brake pressure in the brake cylinders of the vehicle by means of solenoid valves, as disclosed by German patent document DE 36 03 810 A1. In such electric braking systems, synchronization, repeatability, precision of set brake pressures and safety requirements must be high. If an anti-lock braking system (ABS) is available, the pressure modulators or solenoid valves (MV) used for pressure control must also function very rapidly, so that the desired pressures can be achieved within the usual ABS-control cycle of 2 to 5 seconds.
In the simplest case, the solenoid valves can be switching open/close valves which are actuated electrically and are either fully open or fully closed. The braking pressure can be increased, maintained or lowered using such valves. The changes in the brake pressure occur in steps.
It is also possible to use pressure modulators which are able to set the desired pressures in an analogous manner. However, these are relatively expensive and not especially rapid.
Stepping solenoid valves which are actuated electrically with a frequency that is higher than their own frequency, e.g., 30 Hz, can also be used, as disclosed in European Patent EP 0 014 369 B1. In this manner, the solenoid valve can be operated within a relatively narrow working range in the form of a continuously adjustable throttling point. The comfort of control with such a valve is greater than with a purely open/close valve and the wear of the valve seats is reduced. However, considerable problems occur in the control because such a solenoid valve is subject to delay time, greatly prone to hysteresis, has non-linear operating characteristics, and is sensitive to the limitations of manufacturing tolerances and aging.
Additional problems in brake pressure control in a utility vehicle result from the non-linearity of the transmission path, i.e., the pipe and hose lines, and the volume to be controlled, i.e., the brake cylinder. Thus, for example, pressure oscillations may occur in the presence of a large gradient in volume flow. Additional non-linearities result from the fact that the brake cylinder has a highly changeable volume. Furthermore, temperature equalization effects occur in the brake cylinder even when the solenoid valve is closed, i.e., the brake cylinder pressure changes slowly with temperature changes, even when there is no change in volume.